This invention relates to thermoplastic polymers having tetrafluoroethylene units and perfluoro alkyl vinyl ether units, mixtures of such polymers that contain low molecular weight and high molecular weight components, and to processes and articles that employ such polymers.
Copolymers of tetrafluoroethylene (TFEs below) and perfluoro alkyl vinyl ethers having from 1 to 4 carbon atoms in the alkyl moiety (PAVEs below), in particular perfluoro n-propyl vinyl ether (PPVEs below) have been known for a long time. Such copolymers are commercially available under the designation xe2x80x9cPFAxe2x80x9d. At a PAVE copolymer content of about 2% by weight and greater, these partially crystalline copolymers have excellent technical performance, for example exceptional chemical stability, combined with high service temperatures. They can be processed from the melt as thermoplastics, for example by compression molding, extruding or injection molding. Preferred applications are, inter alia, extruded pipes, tubes and cable sheathing. Processing from the melt takes place at temperatures of from 350 up to 450xc2x0 C. Under these conditions, both thermal and mechanical degradation occur.
The thermal degradation takes place predominantly via the thermally unstable end groups formed in the polymerization, i.e. from the end of the chain. The mechanism of this degradation is described in more detail in xe2x80x9cModern Fluoropolymersxe2x80x9d, John Wiley and Sons, 1997, K. Hintzer and G. Lxc3x6hr, Melt Processable Tetrafluoroethylene-Perfluoropropylvinyl Ether Copolymers (PFA), page 223. The degradation can be substantially suppressed by converting the thermally unstable end groups into stable CF3 end groups by postfluorination, as described, for example in U.S. Pat. No. 4,743,658 and DE-C-19 01 872.
Corrosive gases arise during the thermal degradation, and these considerably impair the quality of the final product by metal contamination or bubble formation, and can corrode tooling and processing machinery. The effect naturally increases with falling molecular weight (lower melt viscosity).
The mechanical degradation during processing takes place through chain breakage, recognizable by the increase of the melt flow index (MFI). It increases as extrusion speed (shear rate) rises. The resultant lowering of molecular weight considerably worsens the mechanical properties, in particular the flexural fatigue strength and other long-term properties, such as long-term failure (stress crack resistance). Keeping the mechanical degradation within acceptable limits places corresponding limitations on processing conditions. This applies in particular to the extrusion speed for pipes, tubes and cable sheathing. At higher extrusion speeds, melt fracture (shark skin) also occurs, as with all thermoplastics. Although it is possible to implement higher extrusion speeds without melt fracture by lowering the molecular weight (higher MFI values), such products do not have adequate mechanical properties. For this reason, PFA products with an MFI value  greater than 15 are currently not on the market.
It is known from WO-A-97/07147 that a marked rise in the extrusion rate is possible, while avoiding melt fracture and with retention of the mechanical properties, with partially crystalline copolymers which consist essentially of TFE and at least 3% by weight of perfluoro ethyl vinyl ether and which have a melt viscosity of not more than 25xc3x97103 Pas at 372xc2x0 C., with the proviso that the melt viscosity may exceed this value if the content of the ether mentioned exceeds 10% by weight. The perfluoro ethyl vinyl ether is, however, difficult to obtain, and therefore all of the marketed products contain PPVE, which is easily obtainable industrially and is also preferred for the present invention.
A PFA has now been found which has good melt processability and which contains at least one high-molecular-weight PFA with an MFIxe2x89xa615, preferably from 0.01 to 15, and at least one low-molecular-weight PFA with MFIxe2x89xa730. The mixtures of the invention are particularly useful in applications where chemical resistance and high temperature resistance are important.
The invention therefore relates to mixtures of thermoplastic fluoropolymers essentially comprising units of TFE and subordinate amounts of units of one or more PAVEs having from 1 to 4 carbon atoms in the alkyl moiety and a total concentration of from 0.5 to 10 mol %, the mixture comprising A) at least one low molecular weight component with an MFIAxe2x89xa730 and B) at least one high molecular weight component with an MFIBxe2x89xa615. These components are selected in such a way that the ratio of the MFIA of component A) to the MFIB of component B) is in the range from 80 to 2500, preferably in the range of from 240 to 750.
xe2x80x9cEssentially comprising units of TFE and of a PAVExe2x80x9d means that small amounts, up to about 5 mol %, of other fluoromonomers not containing hydrogen, such as hexafluoropropene or chlorotrifluoroethylene, are not to be excluded. The composition of the copolymer of the two components may differ within the limits mentioned above.
The mixing ratio of high- and low-molecular-weight components may vary within wide limits and can be determined for the desired application by means of simple preliminary experiments. The ratio is generally from 10:90 to 90:10, preferably in the range from 25:75 to 75:25 parts by weight and in particular from 60:40 to 40:60 parts by weight.
The invention also relates to a novel low-molecular-weight PFA with an MFIxe2x89xa730, preferably xe2x89xa7120 with preference from 120 to 1000, in particular from 120 to 700, especially from 200 to 600.
Another aspect of the invention relates to mixtures of the novel low-molecular-weight PFA(s) mentioned with the high-molecular-weight PFA(s) mentioned above, the MFI ratio mentioned above corresponding approximately to a molecular weight ratio of the high-molecular-weight to the low-molecular-weight component(s) xe2x89xa73.5, preferably from 3.5 to 10, in particular from 3.5 to 7.
The MFI gives the amount of a melt in grams per 10 min which is extruded from a holding cylinder through a die by the action of a piston loaded with weights. The dimensions of the die, the piston, the holding cylinder and the weights are standardized (DIN 53735, ASTM D-1238). All of the MFIs mentioned here have been measured with a die of diameter 2.1 mm and length 8 mm using a superimposed weight of 5 kg and a temperature of 372xc2x0 C. The values 0.01 and 1000 are practically the limiting values of this measurement method.
For very high MFI values, therefore, it is expedient to reduce the superimposed weight to values down to 0.5 kg, and for very small MFI values to increase it to values up to 20 kg. The MFI values determined in this way are recalculated for a superimposed weight of 5 kg.
The present invention further provides a process for making a shaped article from the mixtures of the invention. This process involves providing the mixture, extruding, compression molding, or injection molding the mixture, and preferably, cooling the mixture to provide a self-supporting shaped article.
Still further the present invention provides shaped articles comprising the mixture. Examples of such articles include molded or extruded goods such as films, pellets, wire and cable insulation, tubes and pipes, containers, vessel liners, and the like.
The novel mixtures may be prepared in a conventional manner, i.e. for example by mixing the pulverulent products, mixing dispersions of the components, or by conducting the polymerization in an appropriate manner (xe2x80x9cstep polymerizationxe2x80x9d) with controlled use of initiator and chain transfer agent, such as short-chain alkanes and haloalkanes, and also hydrogen. An advantageous procedure here is as follows: at the start of the polymerization, for a low desired MFI, relatively little initiator and relatively little chain transfer agent are metered in. These polymerization conditions are changed at the desired juncture in the polymerization, depending on the type of composition by weight to be achieved, for example after 50% of the TFE addition, by metering in further initiator and chain transfer agent, so that the polymer produced as the polymerization continues has the desired high MFI. The desired high MFI may also be created by increasing the temperature during the polymerization. The advantage of this preparation process is that a xe2x80x9cperfectxe2x80x9d mixture of the two components is created in situ.
Preference is given to mixing dispersions of the components and working up the mixture in a manner known per se (U.S. Pat. No. 4,262,101) or advantageously by mechanical precipitation using a homogenizer, followed by agglomeration by petroleum fractions. After subsequent drying, the product is subjected to melt granulation.
Because the two components have very different MFI values, homogeneous mixtures of powders or of melt granules down to the micro range can be produced only with equipment which is relatively highly elaborate. However, homogeneous mixtures are essential for achieving excellent performance.
Compared with a PFA having comparable MFI, the novel mixtures are distinguished by considerably increased extrusion speed without melt fracture. However, as shown by MFI determination before and after processing, this is not at the cost of significantly increased degradation.
The novel mixtures have a noticeably increased zero-shear viscosity and a lower complex viscosity at higher shear rates, compared with a commercially available polymer component with identical MFI.
The PFA with MFIxe2x89xa730 differs from the hitherto conventional grades of PFA in its low molecular weight. It therefore has a relatively large number of labile end groups, which limit the thermal stability of the material. For relatively stringent requirements therefore it is expedient to convert the unstable end groups to stable end groups in a manner known per se by reaction with elemental fluorine (GB A 1 210 794, EP-A-0 150 953 and U.S. Pat. No. 4,743,658). It is expedient here to dilute the fluorine with an inert gas and to use this mixture to treat the dry polymer or polymer mixture. The toxic fluorine is then removed by flushing with inert gas. This same process may be used to post fluorinate the mixtures of the invention.
The success of the postfluorination is checked by IR-spectroscopic determination of the residual carboxyl and/or carbonyl fluoride end groups, as described in U.S. Pat. No. 4,743,658. However, complete fluorination of the end groups is not necessary. Reduction of the thermally unstable end groups (COOH+COF) to from 10 to 15 end groups/106 carbon atoms is sufficient to achieve the desired improvements in properties. This significantly shortens the reaction time and therefore makes the postfluorination more cost-effective.
The novel PFA mixture postfluorinated in this way shows no discoloration, even at 450xc2x0 C. It therefore permits higher processing temperatures and thus a rise in the throughputs in the extrusion of tubes and of sheathing for wires and cables, and also in injection molding. A further advantage of the increased high-temperature resistance is that when production failures occur, the novel PFA mixture remains for a longer residence time at high temperatures without degradation and thus there is no discoloration or bubble formation at elevated temperature and no corrosion of the processing machinery or of the substrates which come into contact with the polymer mixture.
The preferred process for preparing the novel mixtures consists in blending the two components as dispersions, agglomerating these, drying and melt granulation followed by water-treatment (DE-A-195 47 909) of the granules obtained from the melt and, if desired, postfluorination of the same.
The novel mixtures are advantageously suitable for producing thin-walled articles by extrusion or extrusion blow molding and injection molding. The higher processing speeds which are possible here do not have to be obtained at the cost of impairment of properties; on the contrary, the products obtained surprisingly have increased stiffness (increased modulus of elasticity) and yield stress, i.e. the novel mixtures can resist higher mechanical stresses in particular applications, since an increased yield stress means an enlargement of the elastic range of these materials. This makes it possible to create moldings with longer service lives, and this in turn permits the use of tubes with thinner walls.
The polymerization may be carried out by known processes of aqueous free-radical emulsion polymerization (U.S. Pat. Nos. 3,635,926, 4,262,101), or in a non-aqueous phase (U.S. Pat. No. 3,642,742).
The perfluoro propyl vinyl ether content is determined by IR spectroscopy (U.S. Pat. No. 4,029,868).
EP-B-362 868 has already disclosed mixtures of fluoropolymers, including investigation of high-molecular-weight and low-molecular-weight PFA grades. The low-molecular-weight component here is defined by a melt viscosity at 380xc2x0 C. of from 5000 to 280,000 Poise, corresponding to an MFI at 372xc2x0 C. of from 80 to 1.6. It is expressly mentioned here that a melt viscosity of less than 5000 Poise (MFI greater than 80) leads to poor mechanical properties of the mixture. In the mixture described as example in EP-B-362 868, column 4, the mean molecular weights of the PFA grades used differ only slightly, to be specific approximately only by a factor of 1.5, corresponding to the melt viscosities of 8.1xc3x97104 and 1.9xc3x97104 Poise, respectively. Such materials are particularly suitable for thick-walled extruded articles, such as pipes.